Humid tropical forests are often characterized by
large nitrogen (N) pools, and are known to have
large potential N losses. Although rarely measured,
tropical forests likely maintain considerable biological
N fixation (BNF) to balance N losses. We
estimated inputs of N via BNF by free-living microbes
for two tropical forests in Puerto Rico, and
assessed the response to increased N availability
using an on-going N fertilization experiment.
Nitrogenase activity was measured across forest
strata, including the soil, forest floor, mosses, canopy
epiphylls, and lichens using acetylene (C2H2)
reduction assays. BNF varied significantly among
ecosystem compartments in both forests. Mosses
had the highest rates of nitrogenase activity per
gram of sample, with 11 ± 6 nmol C2H2 reduced/g
dry weight/h (mean ± SE) in a lower elevation
forest, and 6 ± 1 nmol C2H2/g/h in an upper elevation
forest. We calculated potential N fluxes via
BNF to each forest compartment using surveys of
standing stocks. Soils and mosses provided the
largest potential inputs of N via BNF to these ecosystems.
Summing all components, total background
BNF inputs were 120 ± 29 lg N/m2/h in
the lower elevation forest, and 95 ± 15 lg N/m2/h
in the upper elevation forest, with added N significantly
suppressing BNF in soils and forest floor.
Moisture content was significantly positively correlated
with BNF rates for soils and the forest floor.
We conclude that BNF is an active biological process
across forest strata for these tropical forests,
and is likely to be sensitive to increases in N
deposition in tropical regions.

The affinity of iron oxides and hydroxides for phosphorus is thought to contribute to
phosphorus limitation to net primary productivity in humid tropical forests on acidic, highly
weathered soils. Perennially warm, humid conditions and high biological activity in these soils can
result in fluctuating redox potential that in turn leads to considerable iron reduction in the presence
of labile carbon and humic substances. We investigated the effects of reducing conditions in
combination with the addition of labile carbon substrates (glucose and acetate) and an electron
shuttle compound on iron reduction and phosphorus release in a humid tropical forest soil. Glucose
or acetate was added to soils as a single dose at the beginning of the experiment, and as pulsed
inputs over time, which more closely mimics patterns in labile carbon availability. Iron reduction
and phosphorus mobilization were weakly stimulated by a single low level addition of carbon, and
the addition of the electron shuttle compound with or without added carbon. Pulsed labile carbon
additions produced a significant increase in soil pH, soluble iron, and phosphorus concentrations.
Pulsed labile carbon inputs also promoted the precipitation of ferrous hydroxide complexes which
could increase the capacity for P sorption, although our results suggest that rates of P solubilization
exceeded re-adsorption. Plant and microbial P demand are also likely to serve as an important sinks
for released P, limiting the role of P re-adsorption. Our results suggest that reducing conditions
coupled with periodic carbon inputs can stimulate iron reduction and a corresponding increase in
soil phosphorus mobilization, which may provide a source of phosphorus to plants and microorganisms
previously undocumented in these ecosystems.